1. Field of the Invention
The present invention relates to a method and an apparatus for manufacturing a liquid crystal display substrate, and more particularly to a method of manufacturing a liquid crystal display substrate having a drive circuit for driving switching elements, and apparatus and method for evaluating semiconductor crystals.
2. Description of the Related Art
LCDs (Liquid Crystal Displays) having TFTs (Thin-Film Transistors) used as switching elements can display images of very high quality. Therefore, they are useful and attracting much attention.
FIG. 1 is a sectional view of the substrate of an LCD. As shown in the figure, TFTs 1b are formed on the center parts of a glass substrate 1a. Pixel electrodes 1c are formed on the center region of the substrate 1a, spaced apart from the TFTs 1b, and electrically connected to the drains of the TFTs 1b, respectively. Each TFT 1b and the pixel electrode associated with the TFT 1b constitute a square pixel unit U having sides of hundreds of microns. Hundreds of thousands of pixel units U are arranged on the glass substrate 1a.
The LCD substrate further comprises a transparent electrode 1d and a transparent substrate 1f. The transparent electrode 1d, which is used common to the pixel units U, is formed on the transparent substrate 1f. The substrate if is so positioned that the electrode 1d is spaced apart by a predetermined distance from the center part of the glass substrate 1a, opposing the pixel units U. The gap between the transparent electrode 1d and the center part of the glass substrate 1a is filled with liquid crystal 1e. The center part of the glass substrate 1a, the pixel units U, the liquid crystal 1e, the transparent electrode 1d, and the transparent substrate if constitute a pixel section 10.
FIG. 2 is a perspective view of the LCD substrate. As shown in this figure, the LCD substrate further comprises packaged IC chips 11 on the exposed portion of the glass substrate 1a, and around the sides of the pixel section 10. The IC chips 11 form a gate driver. The terminals of the IC chips 11 are coupled to scanning electrode lines which in turn are connected to the gate and drain electrodes of the pixel units U.
How to manufacture the LCD substrate shown in FIGS. 1 and 2 will be described. First, a film of hydrogenated amorphous silicon (a--SiH) is formed on the glass substrate 1a by means of, for example, plasma CVD method. Then, various films and layers are formed on the amorphous silicon film and subjected to various processes (including etching), forming a number of TFTs 1b, pixel electrodes 1c connected to the TFTs 1b, and scanning electrode lines (not shown) connected to the pixel electrodes 1c. A number of pixel units U, forming a block, are thereby prepared. Next, packaged IC chips 11 are mounted on the glass substrate 1a, around the block of pixel units U. The IC chips 11 are aligned with the respective scanning electrode lines of the pixel units U and electrically connected thereto subsequently. Thereafter, the transparent substrate 1f, with the transparent electrode 1d formed on it, is positioned in place, so that the electrode 1d is spaced apart by a predetermined distance from the center part of the glass substrate 1a, opposing the pixel units U. Liquid crystal 1e is applied into the gap between the electrode 1d and the center part of the glass substrate 1a. As a result, the pixel section 10 is fabricated.
There is a demand for a TFT-LCD which has a large display screen and which can display color images of high quality. To meet the demand, it is necessary to use a gate driver which has as many as 400 lines, and a source driver which has as many as 1920 lines. Inevitably it requires a large number of machine-hours to align these lines with the scanning electrode lines of the pixel units, respectively, before electrically connected to the scanning electrode lines. This is one factor of raising the price of the TFT-LCD.
To reduce the price of the TFT-LCD it is desirable that the lines for connecting the gate and source drivers to the pixel section be formed simultaneously with the switching elements of the pixel section. Once the lines of the drivers are so formed, the driver will be connected to the pixel section when they are formed on that portion of the glass plate which extends outside the pixel section. This method of forming the lines simultaneously with the switching elements of the pixel section would save many machine-hours and is a very useful method.
The TFTs of the pixel unit, which are used to display an image, need not operate at high speed. In contrast, it is required of the gate and source drivers, both used for switching the TFTs fast, should operate at much higher speed than the TFTs. In other words, the drivers must have the same operating speed as IC chips. The drivers must therefore comprises a semiconductor film of polycrystalline silicon which have higher field-effect mobility than amorphous silicon.
To form a polycrystalline silicon film on a glass substrate, it is necessary to perform pressure-reduced CVD method at 600.degree. C. or more, at which temperature a substrate of cheap glass will warp. Hence, the substrate must be made of expensive glass such as a quartz glass which would not warp at such high temperatures. This will ultimately raise the manufacturing cost of the TFT-LCD.
A method of forming a polycrystalline silicon film on a substrate of cheap glass, without warping the substrate, has been proposed to avoid an increase in the manufacturing cost of the TFT-LCD. The method comprises the step of forming a large film of hydrogenated amorphous silicon (hereinafter referred to as "a--Si:H") by, for example, plasma CVD in an atmosphere of about 300.degree. C., and the step of applying a laser beam onto the a--Si:H film, thereby raising the surface temperature thereof to about 1200.degree. C. and thus converting the surface region of the a--Si:H film to a polycrystalline silicon film. The polycrystalline silicon film, thus formed can be used as the semiconductor film of the gate and source drivers.
The a--Si:H film formed can be large and of high quality since it is formed at a temperature as low as about 300.degree. C. The application of a laser beam onto the a--Si:H film (i.e., laser annealing) lasts but an extremely short time, for example only 23 nsec in the case of a KrF laser beam and the heat does not reach the glass substrate. Hence, the substrate need not be so heat resistant and can be made of cheap glass. Furthermore, since the a--Si:H film is formed at low temperature and ever a large area, the polycrystalline silicon film is equally large because it is the region of the a--Si:H film, which has been crystallized with a laser beam. This helps to produce a large-screen LCD of transmission type.
When applied with a laser beam, the a--Si:H film generates hydrogen, which may damage the film. To prevent such a damage, many ideas have been propose in respect of methods of applying laser beams, the energy of each laser beam to apply, and the like. The present inventors has been studying a method of applying laser beam pulses onto a strip-shaped target region of a rectangular a--Si:H film, which extends along at least one side of the film, thereby crystallizing the target regions. (Drive circuit sections of an LC substrate may be formed in the strip-shaped region thus crystallized.) This method, however, has the following problems.
FIGS. 3A and 3B are diagrams, each illustrating the relationship between the positions of two adjacent laser beams applied to an a--Si:H film, on the one hand, and the field-effect mobility in the film, on the other. Each laser beam has an intensity which is uniformly distributed in the direction parallel to the a--Si:H film and which diverges toward the a--Si:H film.
In the case where the adjacent two laser beams do not overlap at all, as shown in FIG. 3A, no laser-beam energy is applied to that region of the film which lies between the regions thereof irradiated with the laser beams. This region therefore remains not crystallized, whereas the irradiated regions are crystallized. Polycrystalline silicon and a--Si:H and polycrystalline silicon have field-effect mobility of 30 to 600 cm.sup.2 /V s and 0.3 to 1 cm.sup.2 /V s, respectively. The field-effect mobility of polycrystalline silicon is two orders of magnitude greater than that of a--Si:H. An a--Si:H region is formed in the polycrystalline silicon region, the drive circuit sections formed in the strip-shaped region crystallized with laser beams successively applied to the a--Si:H film will greatly differ in their operating characteristic.
In the case where the adjacent two laser beams overlap at least in part, as shown in FIG. 3B, a portion of the a--Si:H film is irradiated two times to have field-effect mobility higher than the remaining portion of the a--Si:H film, despite that each laser beam has an intensity which is uniformly distributed in the direction parallel to the a--Si:H film. This is because polycrystalline silicon and a--Si:H have melting points of 1414.degree. C. and 1000.degree. C., respectively, so that the sizes of their grains vary. The polycrystalline silicon film cannot be uniform in terms of field-effect mobility (In FIG. 3B, the irregular portion has a higher mobility as shown by a projection curve, typically, but may has a lower mobility.)